Added testing with tox
Exported formatter
Deduplicate units from candidates search

Previous to this commit, _parse_candidate was returning an iterable of
possible units, including prefixes and suffixes combinations.
If multiple values where returned, the calling functions were issuing
a warning to inform the user that there are multiple matches for a
given string. For example, if a user defines a new unit ms, it could
be mistaken with prefix=mili, unit=second. However, as the registry
stores also prefixed units many times a duplication is not a real
problem (eg. prefix=kilo, unit=gram and unit=kilogram).
To avoid issuing false warnings a deduplication step was added to
parse candidates. In the future, a more efficient alternative will be
to block adding colliding units to the registry.
tox
tox
Initial refactoring of the NumPy testing
Begin refactoring of the NumPy Testing infrastructure
In place conversion should not create a new object when source and destination units are equal